Phosphor-converted light emitting diode (“LED”) devices are useful for generating light output having actual and perceived spectral characteristics that differ from the actual spectral characteristics of the LEDs themselves. For example, the advent of blue LEDs was a key development in the quest for LED devices emitting apparently white light, as potential replacements for incandescent and fluorescent bulbs. Blue LEDs have been integrated with yellow phosphors to emit blue and yellow photons in ratios that are perceived by the human eye as white light. Although these photonic emissions do not span the complete visible spectrum and therefore are not actually equivalent to sunlight, they appear to be white and thus may be effectively utilized, for example in lighting applications. Since LED devices may convert electricity into photonic emissions more efficiently than incandescent and fluorescent bulbs, the potential benefits of LED use for lighting and other applications in terms of energy conservation are great. Further, as solid state devices, LED devices have a larger average lifetime of use than and often are more resistant to physical damage than are conventional incandescent and fluorescent bulbs.
Phosphor-converted LED devices typically emit photons having at least two discrete wavelengths, which are generated by at least two different sources that are located close to, but not in identical positions as, each other. One source is electroluminescent radiation from the LED itself; another is luminescent radiation from the phosphor, as stimulated by radiation from the LED. Unfortunately, the non-unity of both the physical positioning and the functional operation of these photonic sources generally results in non-uniformity in the additive photonic emissions from conventional phosphor-converted LED devices, producing an unwanted wide white color bin spread.
As an example, the structure of a conventional phosphor-converted LED device may include an LED that is overlaid by a selected phosphor. In an example of operation, the electroluminescent emissions from the LED at one wavelength are partially intercepted by the phosphor, resulting in stimulated luminescent emissions from the phosphor that are usually at a longer wavelength. Photons emitted by the LED at a first wavelength and by the phosphor at a second wavelength are then additively emitted from the phosphor-converted LED device. It is appreciated by those skilled in the art that the LED may be designed to emit blue photons, and the phosphor may be designed to emit yellow photons, in ratios where the additive output is perceived by the human eye as white light.
In an example of fabricating a phosphor-converted LED device, the phosphor is dispersed in a suitable encapsulant in a liquid phase and then deposited onto the LED. The phosphor generally migrates downward in the encapsulant following deposition, as the encapsulant cures to a solid form. This migration often leads to uneven layering of the phosphor over the LED, which results in a phosphor-converted LED device producing a wide white color bin spread. As an example of problems associated with this conventional fabrication method, if the shape of the LED is a rectangular prism, then the phosphor may sink to the bottom of the phosphor-encapsulant dispersion to the point that further migration of the phosphor is partially impeded by the LED itself. As this impedance develops while the phosphor dispersion cures, the phosphor may become unevenly distributed across the upper surface of the LED rectangular prism onto which it sinks. In particular, portions of the sinking phosphor that clear the outer edges of the top surface of the LED may further sink below that surface. As a result, the thickness of the phosphor layer may be decreased near the outer edges. Upon stimulation of electroluminescent emissions from the LED itself, this decreased thickness may result in reduced capacity by the phosphor near the outer edges to convert the photons emitted by the LED by stimulated emissions. This reduced capacity imbalances the desired ratio between blue and yellow photons emitted from the phosphor-converted LED device in regions over the outer edges, because the yellow photonic emissions there are reduced. Hence, a wide white color bin spread may result and a blue halo may be generated in the photonic output of the phosphor-converted LED device, roughly conforming to the locations of the thin regions in the phosphor near such outer edges. This blue halo constitutes a non-uniformity in the light output from the phosphor-converted LED device that may be both aesthetically and functionally undesirable in use of the device.
Therefore, as phosphor-converted LED devices are implemented for diverse end use applications, there is a continuing need to provide new phosphor-converted LED device structures generating photonic emissions of improved uniformity.